Fiber-based adsorbents having high adsorption capacities for recovering dissolved metals and methods thereof

ABSTRACT

A fiber-based adsorbent and a related method of manufacture are provided. The fiber-based adsorbent includes polymer fibers with grafted side chains and an increased surface area per unit weight over known fibers to increase the adsorption of dissolved metals, for example uranium, from aqueous solutions. The polymer fibers include a circular morphology in some embodiments, having a mean diameter of less than 15 microns, optionally less than about 1 micron. In other embodiments, the polymer fibers include a non-circular morphology, optionally defining multiple gear-shaped, winged-shaped or lobe-shaped projections along the length of the polymer fibers. A method for forming the fiber-based adsorbents includes irradiating high surface area polymer fibers, grafting with polymerizable reactive monomers, reacting the grafted fibers with hydroxylamine, and conditioning with an alkaline solution. High surface area fiber-based adsorbents formed according to the present method demonstrated a significantly improved uranium adsorption capacity per unit weight over existing adsorbents.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/510,515, filed Jul. 22, 2011, the disclosure of which is herebyincorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT

This invention was made with government support under Contract No.DE-AC05-00OR22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to fiber-based adsorbents for the recoveryof uranium and other dissolved metals from aqueous solutions.

Uranium is dissolved in seawater across the world's oceans at a uniformconcentration of approximately 3.2 ppb with the total amount of uraniumin seawater of approximately 4-5 billion metric tons, which is about1000 times larger than the amount of uranium in terrestrial ores. Mostof the dissolved uranium in seawater exists as uranyl tricarbonate ion(UO₂ (CO₃)₃ ⁴⁻). Development of uranium adsorbents has been researchedsince the 1960s including work on hydrous titanium oxide and other metaloxides, however the adsorption capacity (about 0.1g-Uranium/kg-adsorbent) and mechanical strength of these materials weredeemed too low for practical use. In the 1980s efforts shifted towardsdeveloping uranium adsorbents containing organic materials including theamidoxime group which was found to be particularly promising forcomplexing uranyl ions in seawater. Polymeric beads containing theseamidoxime groups were initially evaluated, however this approach wasabandoned due to practical handling issues.

Fiber-based adsorbents containing amidoxime groups have been researchedsince the 1980s. Early versions were based on polyacrylonitrile fiberswhich were reacted with hydroxylamine to form amidoxime groups, howeversince these groups were formed evenly in the fiber, the mechanicalstrength of the fiber was insufficient to survive in the sea. Toalleviate this issue, graft co-polymerization of polyolefin fibers(e.g., polyethylene and polypropylene) with polymerizable monomers wasused to produce either nonwoven or continuous fiber braided adsorbents.This process involved co-grafting nitrile groups (e.g., acrylonitrile)and hydrophilic groups (e.g., methacrylic acid) ontopreviously-irradiated polyolefin fibers having a diameter of at least 15microns to form grafted side chains, then reacting the nitrile groupswith hydroxylamine (NH₂OH) to convert them to amidoxime groups followedby alkaline (e.g., KOH) conditioning.

The nonwoven adsorbents were investigated for many years; however thesematerials are constructed using short, discontinuous, thermallyspun-bonded fibers which have relatively poor mechanical strengthcompared to continuous fiber forms. In particular, nonwoven adsorbentswere evaluated in several seawater experiments and demonstrated uraniumadsorption capacities of about 1.5 g-Uranium (U)/kg-adsorbent after 30days immersion in seawater. Due to their low mechanical strength, thenonwoven adsorbents necessitated their incorporation into large sandwichstacks composed of spacer nets and stack holders placed on large, heavyfloating frames which eventually proved too costly for implementation.In addition, the sandwich stacks containing the nonwoven adsorbentprevented good accessibility to the seawater resulting in loweradsorption capacities compared to braided adsorbents. These braidedadsorbents are composed of continuous polyethylene fibers that arebraided around a porous polypropylene float that can be made into longlengths. The braided adsorbent is currently the material of choice foruranium adsorbents due to the favorable balance of properties includinghigh mechanical strength, elongation-to-break, durability, low cost,chemical resistance (i.e., acids, bases, solvents), as well as theirease of placement and retrieval from the sea. However, the uraniumadsorption capacity of the braided polyethylene adsorbents is relativelylow, at 1.5 g-U/kg-adsorbent after 30 days immersion in seawater, to becost effective for implementation.

Accordingly, there remains a continued need for an improved fiber-basedadsorbent having an increased adsorption capacity for the recovery ofuranium and/or other dissolved metals from seawater, river water andother aqueous solutions.

SUMMARY OF THE INVENTION

In a first aspect of the invention, a fiber-based adsorbent and arelated method of manufacture are provided. The fiber-based adsorbentand the related method include grafted polymer fibers having anincreased surface area per unit weight to improve the adsorption ofdissolved metals, for example uranium. The polymer fibers include acircular cross-section having a mean diameter of less than 15 microns insome embodiments, optionally less than about 1 micron. In otherembodiments, the polymer fibers include a non-circular cross-section,optionally defining multiple gear-shaped, lobe-shaped or winged-shapedprojections along the length of the polymer fibers.

In a first embodiment, a method for manufacturing an adsorbent materialincludes providing polyolefin fibers including a circular cross-sectionhaving a mean diameter of less than about 15 microns, optionally lessthan about 1 micron, and further optionally about 0.25 microns, exposingthe polyolefin fiber to ionizing radiation, co-grafting polymerizedmonomers containing nitrile and hydrophilic groups to form grafted sidechains, converting the nitrile groups in the grafted side chains intoamidoxime groups, and conditioning the resulting structure with analkaline solution. Co-grafting is optionally performed while thepolyolefin fiber is exposed to ionizing radiation, or alternativelyafter the polyolefin fiber is exposed to ionizing radiation. Theadsorbent, and in particular, the grafted side-chains, are adapted tocomplex uranium ions from an aqueous solution, optionally having auranium adsorption capacity of at least about 50 g-U/kg-adsorbent froman aqueous solution including 6-7 ppm uranyl nitrate hexahydrate.

In a second embodiment, a method for manufacturing an adsorbent materialincludes providing polyolefin fibers including a non-circularmorphology, exposing the polyolefin fiber to ionizing radiation,co-grafting polymerized monomers containing nitrile and hydrophilicgroups to form grafted side chains, converting the nitrile groups in thegrafted side chains into amidoxime groups, and conditioning theresulting structure with an alkaline solution. The non-circularmorphology can include a lobed-morphology, a core/shell or sheathedmorphology, or any morphology having at least one surface projection orsurface recess along the polyolefin fiber. For example, the fibercross-section can be trilobial-shaped, gear-shaped, flower-shaped, andthe fiber can be hollow or solid as desired. Co-grafting is optionallyperformed while the polyolefin fiber is exposed to ionizing radiation,or alternatively after the polyolefin fiber is exposed to ionizingradiation. The adsorbent, and in particular, the grafted side chains,are adapted to complex uranium ions from an aqueous solution, optionallyhaving a uranium adsorption capacity of at least about 50g-U/kg-adsorbent from an aqueous solution including 6-7 ppm uranylnitrate hexahydrate.

In the above embodiments, the polyolefin fiber can include, for example,polyethylene or polypropylene, and the fiber can also include polyamide,polyester, polyvinyl alcohol, polyvinyl chloride,polytetrafluoroethylene-ethylene copolymer, polyacrylonitrile, andcombinations thereof. The selected fiber can form a woven fabric, abraided fabric, a knitted fabric or non-woven fabric, or any othertextile form whether now known or hereinafter developed. To convert thenitrile groups into amidoxime groups, the above methods can includereacting the nitrile groups in the grafted side chains withhydroxylamine, or alternatively, hydroxylamine derivatives, hydrazine,hydrazine derivatives, N-methylhydroxylamine, acetohydroxamic acid,N-benzylhydroxylamine hydrochloride, hydroxyurea, tert-butyln-hydroxycarbamate, sym-diphenylhydrazine, methylhydrazine sulfate,phenylhydrazine or hydrochloride. Also in the above embodiments, thenitrile groups can include, for example, acrylonitrile, vinylidenecyanide, crotonnitrile, methacrylonitrile, chloroacrylonitrile,2-cyanomethacrylate or 2-cyanoethylacrylate, and the hydrophilic groupscan include, for example, methacrylic acid, acrylic acid, itaconic acid,2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, allyl alcohol,polyethylene glycol acrylate, polyethylene glycol methacrylate,polyethylene glycol diacrylate, polyethylene glycol dimethacrylate,N-vinylpyrrolidone, acrylamide, sulfonic acid group, carboxylic acidgroup, phenolic hydryoxyl group or phosphonic acid group.

In a second aspect of the invention, a method is provided for forming afoam-based adsorbent to recover metal ions from seawater and otheraqueous solutions. The method includes providing an open-cell orclosed-cell polyolefin foam, exposing the polyolefin foam to ionizingradiation, co-grafting polymerizable monomers containing nitrile groupsand hydrophilic groups to form grafted side chains, converting thenitrile groups in the grafted side chains into amidoxime groups, andconditioning the foam with an alkaline solution to obtain a foam-basedadsorbent capable of complexing metal ions from seawater or otheraqueous solution. Co-grafting is optionally performed while thepolyolefin foam is exposed to ionizing radiation, or alternatively afterthe polyolefin foam is exposed to ionizing radiation.

In a third aspect of the invention, a method is provided for forming apowder-based adsorbent to recover metal ions from seawater and otheraqueous solutions. The method includes providing a powder includingpolyolefin granules, exposing the granules to ionizing radiation,co-grafting polymerizable monomers containing nitrile groups andhydrophilic groups to form grafted side chains, converting the nitrilegroups in the grafted side chains into amidoxime groups, andconditioning the powder with an alkaline solution to obtain apowder-based adsorbent capable of complexing metal ions from seawater orother aqueous solution. Co-grafting is optionally performed while thepolyolefin powder is exposed to ionizing radiation, or alternativelyafter the polyolefin powder is exposed to ionizing radiation.

These and other features and advantages of the present invention willbecome apparent from the following description of the invention, whenviewed in accordance with the accompanying drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 are polymerizable monomers for forming grafted side chainscontaining nitrile groups in accordance with the present invention.

FIG. 2 are dinitrile compounds for forming grafted side chainscontaining nitrile groups in accordance with the present invention.

FIG. 3 are nitrile containing small molecules for forming grafted sidechains in accordance with the present invention.

FIG. 4 are monomers for radical polymerization in accordance with thepresent invention.

FIG. 5 depicts a first exemplary chemical modification into nitrilegroups.

FIG. 6 depicts a second exemplary chemical modification into nitrilegroups.

FIG. 7 depicts nitrile introduction by alkylation of ester, ketone oralcohol.

FIG. 8 depicts the addition of a co-monomer to the grafting solution.

FIG. 9 are cyanuric chloride core, nitrile containing small moleculesfor forming grafted side chains in accordance with the presentinvention.

FIG. 10 are hydroxylamine alternatives for converting nitrile groupsthat can also selectively complex uranyl ions and other metal ions froman aqueous solution in accordance with the present invention.

FIG. 11 is a graph illustrating the increase in surface area per unitmass as the fiber diameter is reduced.

FIG. 12 is a first micrograph of high surface area fibers having anislands-in-the-sea morphology for use in accordance with the presentinvention.

FIG. 13 is a second micrograph of high surface area fibers having anislands-in-the-sea morphology for use in accordance with the presentinvention.

FIG. 14 is a micrograph of high surface area fibers having a trilobalmorphology for use in accordance with the present invention.

FIG. 15 is a micrograph of a high surface area fiber having a hollowgear shape morphology for use in accordance with the present invention.

FIG. 16 is a micrograph of a high surface area fiber having a flowershape morphology for use in accordance with the present invention.

FIG. 17 is a micrograph of a high surface area fiber having aworm-shaped or winged-shaped morphology for use in accordance with thepresent invention.

FIG. 18 is a micrograph of high surface area fibers having a solidtrilobal gear shape for use in accordance with the present invention.

FIG. 19 is a micrograph of high surface area fibers having a hollowtrilobal gear shape for use in accordance with the present invention.

FIG. 20 is a micrograph of high surface area fibers having a smalldiameter sheath/core shape for use in accordance with the presentinvention.

FIG. 21 is an illustration of 4DG™ high surface area fibers for use inaccordance with the present invention.

FIG. 22 is a micrograph of an Allasso® high surface area fiber for usein accordance with the present invention.

FIG. 23 is a micrograph of high surface area fibers having a wingedshape for use in accordance with the present invention.

FIG. 24 is a graph illustrating % degree of grafting for selected highsurface area fibers.

FIG. 25 is a plot of the uranium adsorption capacity versus the % degreeof grafting for selected high surface area adsorbents as compared to anexisting non-woven adsorbent.

FIG. 26 is a table illustrating the uranium adsorption capacity and the% degree of grafting for selected high surface area adsorbents ascompared to an existing non-woven adsorbent.

DETAILED DESCRIPTION OF THE CURRENT EMBODIMENTS

The invention as contemplated and disclosed herein includes high surfacearea fiber-based adsorbents having an increased adsorption capacity perunit weight for the recovery of metals, for example uranium, titanium,vanadium, gold, platinum, palladium, silver, rare earth metals, mercury,chromium, cadmium, lead, cobalt, zinc, copper, iron, nickel, and anyother suitable metals, from seawater, river water and other aqueoussolutions. As explained in greater detail below, the fiber-basedadsorbents of the present invention are generally formed by irradiatinghigh surface area polymer fibers, subsequently or simultaneouslyco-grafting polymerizable monomers containing nitrile groups andhydrophilic groups to form grafted side chains, converting the nitrilegroups into grafted amidoxime groups, and conditioning the graftedfibers with an alkaline solution. The resulting fiber-based adsorbentsare capable of selectively complexing uranyl ions and other metal ionsfrom seawater or other aqueous solutions. The uranium can be eluted fromthe adsorbent by treating with mineral acids (e.g., HCl), organic acids(e.g., tartaric acid), sodium carbonate or sodium bicarbonate.Regeneration of the adsorbent for subsequent re-use can be accomplishedby KOH or other alkaline conditioning after acid elution has beencompleted.

I. Irradiating High Surface Area Polymer Fibers

The method for producing a fiber-based adsorbent generally includesexposing high surface area polymer fibers to ionizing radiation to formfree radicals on the polymer fibers. Suitable polymer fibers can includepolyolefin fibers, for example polyethylene and polypropylene, as wellas polyamide, polyester, polyvinyl alcohol, polyvinyl chloride,polytetrafluoroethylene-ethylene copolymer, and mixtures thereof.

The selected polymer fibers can form a woven fabric, a braided fabric, aknitted fabric, or a nonwoven fabric, or other textile form whether nowknown or hereinafter developed. In addition, the selected polymer fiberscan include a circular cross-section or a non-circular cross-section,and can be hollow or solid as desired. Fibers having a circularcross-section can define a mean diameter of less than 15 microns,optionally less than about 1 micron, further optionally about 0.25microns. Fibers having a non-circular cross-section can include alobe-shaped morphology, a wing-shaped morphology, a flower-shapedmorphology, a gear-shaped morphology, or any morphology having at leastone surface projection or surface recess along the polymer fiber asdiscussed more fully in Part IV below.

In the present embodiment, suitable ionizing radiation includes gammaray radiation, electron beam radiation and x-ray radiation, optionallybetween about 10 kGy and about 500 kGy, inclusive. In addition, theirradiated polymer fibers are generally sealed within an inertenvironment to preserve the free radicals prior to grafting. Forexample, the polymer fibers can be sealed within a nitrogen environment,optionally at subzero temperatures, to prevent oxygen from reacting withthe newly formed free radicals. While irradiation is typically conductedseparately from graft polymerization, exposure to ionizing radiation canalternatively occur in the presence of the grafting monomers in liquidor vapor form while under inert conditions. For example, the method ofthe present invention can include simultaneously irradiating a highsurface area polymer fiber while in the presence of polymerizablemonomers in liquid form.

II. Co-Grafting Polymerizable Monomers

The present method additionally includes co-grafting polymerizablemonomers onto the irradiated trunk polymers to form grafted side chainsthroughout the fiber volume. The polymerizable monomers can includemonomers containing nitrile groups and hydrophilic groups containedwithin or without a solvent. Alternatively, the monomers containingnitrile groups and hydrophilic groups can be contained within anaqueous-based emulsion comprising a suitable surfactant. The surfactantcan consist of an anionic surfactant, a cationic surfactant, an amphoricsurfactant, a non-ionic surfactant or a mixture thereof. The monomerscontaining nitrile groups can include, for example, acrylonitrile,vinylidene cyanide, crotonnitrile, methacrylonitrile,chloroacrylonitrile, 2-cyanomethacrylate, 2-cyanoethylacrylate andmixtures thereof. The monomers containing hydrophilic groups caninclude, for example, methacrylic acid, acrylic acid, itaconic acid,2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, allyl alcohol,polyethylene glycol acrylate, polyethylene glycol methacrylate,polyethylene glycol diacrylate, polyethylene glycol dimethacrylate,N-vinylpyrrolidone, acrylamide and mixtures thereof. A grafting solutionincluding polymerizable monomers can include one or more solvents, forexample, dimethylsulfoxide, dimethylformamide and mixtures thereof.

Side chains can also be formed by grafting different monomers containingone or more nitriles to the irradiated high surface area polymer fibers.Optional monomers include an alkene group that can react with the freeradicals, and also contain one or more nitriles that will be convertedto the amidoxime. The monomers containing nitrile groups can includeacrylonitrile, vinylidene cyanide, crotonnitrile, methacrylonitrile,chloroacrylonitrile, 2-cyanomethacrylate, 2-cyanoethylacrylate andmixtures thereof. Additional examples of suitable monomers and theirchemical structures are shown in FIG. 1. Additional examples of suitabledinitrile compounds and their chemical structure are shown in FIG. 2.

To increase the range of compounds that can be grafted to the highsurface area fibers, living radical polymerization monomers can bemodified by reaction with nitrile containing small molecules. A list ofexemplary nitrile containing small molecules and living radicalpolymerization monomers are shown in FIG. 3. In a reverse approach,living radical polymerization monomers can be attached to radiationgenerated free radicals on the high surface area fibers. Exemplarymonomers for radical polymerization are shown in FIG. 4. Thesefunctionalized fibers can then be reacted chemically with the nitrilecontaining small molecules listed above. For the cases of glycidylmethacrylate grafted polymers, a chemical modification can introduce thedesired nitrile groups. This chemical modification can take place on theepoxide pending group as shown in FIG. 5. Various amine or hydroxycontaining groups can modify the side chains on glycidyl methacrylate asdepicted in FIG. 3 (nitrile containing small molecules). Sometimes thedinitrile molecules can also form a cyclic form that can also be usedfor metal adsorption. Other acrylates have also been grafted followingthe same procedure as for gycidyl methacrylate as shown in FIG. 6, andcan be modified with the same range of molecules depicted in FIG. 3(nitrile containing small molecules). In the cases of acrylates such asmethyl acrylate, ethyl acrylate, methyl methacrylate, butylmethacrylate, ethyl methacrylate and glycidyl methacrylate, the nitrilesor desired fuctionalization can be introduced by alkylation of the esterto ketone or in some cases alcohol. This is illustrated in FIG. 7.Stable anions can be formed from the deprotonation of a carbon adjacentto an electron-withdrawing group such as nitriles. Small molecules likeglutaronitrile, 1,5-dicyanopentane, acetonitrile, malononitrile andother similar compounds can be reacted with the polyacrylate graftedmaterial. Hydrophilicity can be added to this material as well by addinga co-monomer like methacrylic acid to the grafting solution as shown inFIG. 8.

Additional nitriles can be added to the high surface area fibers byusing a link, for example cyanuric chloride, to attach the nitrilecontaining molecules before they are attached to the fiber. Theresulting molecule is first reacted with a living radical polymerizationmonomer, and subsequently reacted with the radiation generated radicalsor by the reaction of the link with the already functionalized fiber.Another approach is to modify the same monomers listed in FIG. 9, beforethey are attached to the high surface area fiber, by first convertingthe nitrile functionality to amidoxime with hydroxylamine. If needed,any of these new grafted fibers can be made more hydrophilic with theaddition of various ratios of nitrile containing monomers combined withhydrophilic monomers including: methacrylic acid, acrylic acid, itaconicacid, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate, allylalcohol, polyethylene glycol acrylate, polyethylene glycol methacrylate,polyethylene glycol diacrylate, polyethylene glycol dimethacrylate,N-vinylpyrrolidone, acrylamide and mixtures thereof.

Irradiation induced grafting is not required in all embodiments however.For example, the method of the present invention can include providing apolyacrylonitrile. Where polyacrylonitriles are used, irradiationinduced grafting can be omitted, and the above method can optionallyinclude converting some of the nitrile groups into hydrophilic groupsand other of the nitrile groups into amidoxime groups.

III. Converting Nitrile Groups into Amidoxime Groups and Conditioning

After the grafted side chains are formed on the irradiated high surfacearea fibers, the nitrile groups are converted into amidoxime groups. Inthe present embodiment, the grafted fibers are reacted withhydroxylamine (NH₂OH) to convert the nitrile groups to amidoxime groups.This conversion is followed by alkaline (e.g., KOH, NaOH) conditioningto produce fiber-based adsorbents capable of selectively complexinguranyl ions and other metal ions from seawater or other aqueoussolutions.

While hydroxylamine is used in the present embodiment, hydroxylamine canbe substituted with a range of similar compounds that can alsoselectively complex uranyl ions and other metal ions from an aqueoussolution, including for example seawater. Exemplary hydroxylaminesubstitutes can include hydroxylamine derivatives, hydrazine, hydrazinederivatives, N-methylhydroxylamine, acetohydroxamic acid,N-benzylhydroxylamine hydrochloride, hydroxyurea, tert-butyln-hydroxycarbamate, sym-diphenylhydrazine, methylhydrazine sulfate,phenylhydrazine hydrochloride, with chemical structures depicted in FIG.10.

To reiterate, the method for forming a fiber-based adsorbent generallyincludes irradiating high surface area polymer fibers, grafting withpolymerizable reactive monomers, reacting the grafted fibers withhydroxylamine, and conditioning with an alkaline solution, for examplepotassium hydroxide or sodium hydroxide. The irradiated high surfacearea fibers are generally grafted with reactive monomers containedwithin a solvent, or contained within an aqueous-based emulsioncomprising a suitable surfactant. In addition, the reactive monomersgenerally include, but are not limited to, nitrile and hydrophiliccontaining monomers, including the grafting monomers or mixturesdiscussed above. The fiber-based adsorbents are capable of selectivelycomplexing uranyl ions and other metal ions from seawater or otheraqueous solutions. The uranium is then eluted from the adsorbent bytreating with mineral acids (e.g., HCl), organic acids (e.g., tartaricacid), sodium carbonate or sodium bicarbonate. Regeneration of theadsorbent for subsequent re-use can be accomplished by KOH or otheralkaline conditioning after acid elution has been completed.

IV. High Surface Area Fibers

The surface-area-to-weight-ratio for the fiber-based adsorbent producedaccording to the above method, and consequently its uranium adsorptioncapacity per unit weight, is substantially increased by reducing thediameter of the polymer fibers, by changing the cross-sectional shape ofthe polymer fibers, or a combination of both. As shown in FIG. 11 forexample, a reduction in the diameter of a round fiber from 20 microns toless than 1 micron results in a significant increase in the fibersurface-area-per-unit-weight. Suitable high surface area fibers arecommercially available from Hills Inc. of West Melbourne, Fla., andinclude both round fibers and non-round fibers. As used herein, “highsurface area fibers” include (a) round fibers having a diameter of lessthan 15 microns and (b) non-round fibers with a greatersurface-area-to-weight-ratio than a 15 micron round fiber. For example,round fibers formed according to an island-in-the-sea (I-S) method canachieve a 0.25 micron diameter, resulting in an 8000% increase insurface-area-per-unit-weight when compared with commercially available,20 micron diameter, round fibers formed according to conventionalmelt-spinning processes. In the I-S fiber production method, nanofibersare embedded inside a fiber made of a dissolved polymer (e.g.,polylactic acid). After the nanofibers are formed, the sea polymer isdissolved away to expose the nanofibers, optionally including as many as156,000 nanofibers. For example, FIG. 12 illustrates a 300islands-in-the-sea fiber cross-section, and FIG. 13 illustrates a 600islands-in-the sea, with a 0.5 micron diameter after the sea polymer isdissolved.

As noted above, high surface area fibers can additionally oralternatively include non-round fibers. Non-round fibers can be formedwith a significantly greater surface-area-to-weight ratio compared withround fibers having the same diameter. Fiber morphologies suitable formthe present invention include, but are not limited to, solid or hollowflower shapes (i.e., British flag shape), solid or hollow gear shape,solid or hollow trilobal shape, worm shape, and winged fiber shape.FIGS. 14-23 illustrate several high surface area fibers suitable for usewith the above described method. In particular, FIG. 14 is a micrographof high surface area fibers having a trilobal morphology with an 50% to100% increase in surface area, and FIG. 15 is a micrograph of a highsurface area fiber having a hollow gear-shaped morphology with 100%increase in surface area. FIG. 16 is a micrograph of a high surface areafiber having a flower shape morphology with a 600% increase in surfacearea, and FIG. 17 is a micrograph of a high surface area fiber having aworm shape or winged shape morphology with a 900% increase in surfacearea. FIG. 18 is a micrograph of a high surface area fiber having asolid trilobal gear shape, FIG. 19 is a micrograph of a high surfacearea fiber having a hollow trilobal gear shape, and FIG. 20 is amicrograph of a high surface area fiber having small diametersheath/core fibers. Other high surface area fibers include, but are notlimited to, 4DG™ shaped fibers from Fiber Innovation Technology, Inc. ofJohnson City, Tenn., and Allasso shaped fibers from Allasso Industriesof Morrisville, N.C. For example, FIG. 21 is an illustration of the 4DG™high surface area fibers by Fiber Innovation Technology, Inc., FIG. 22is a micrograph of an Allasso® high surface area fiber by AllassoIndustries, and FIG. 23 is a micrograph of high surface area fibershaving a winged shape.

V. Foam-Based Adsorbents and Powder-Based Adsorbents

Though described in Parts I-IV above as relating to fiber-basedadsorbents, the above described method is also suitable for formingfoam-based adsorbents and powder-adsorbents for the recovery of metalsfrom seawater and other aqueous solutions. The method for formingfoam-based and powder-based adsorbents is similar to the above describedmethod for forming fiber-based adsorbents, except that a) the startingmaterial includes a foam or a powder, or b) the end product (anadsorbent fiber) is ground into a powder. Exemplary starting materialsfor foam-based adsorbents include open cell and/or closed cell foams,optionally polyolefin foams, e.g., polyethylene foam and polypropylenefoam. In addition, exemplary starting materials for a powder-basedadsorbent includes a polyethylene powder, a polypropylene powder and ora polyacrylonitrile powder having a mean granule diameter of about 500microns or less, optionally about 50 microns or less, and furtheroptionally about 5 microns or less. The foam-based adsorbents and thepowder-based adsorbents formed according to the present method includegrafted side chains throughout the entire volume of the respectiveadsorbent. A method for forming a foam-based adsorbent can include thefollowing steps: providing an open-cell or closed-cell polyolefin foam,exposing the polyolefin foam to ionizing radiation, co-graftingpolymerizable monomers containing nitrile groups and hydrophilic groupsto form grafted side chains, converting the nitrile groups in thegrafted side chains into amidoxime groups, and conditioning the foamwith an alkaline solution to obtain a foam-based adsorbent capable ofcomplexing metal ions from seawater or other aqueous solution.Co-grafting is optionally performed while the polyolefin foam is exposedto ionizing radiation, or alternatively after the polyolefin foam isexposed to ionizing radiation. A method for forming a powder-basedadsorbent can include the following steps: providing a powder includingpolyolefin granules or polyacrylonitrile granules, exposing the granulesto ionizing radiation, co-grafting polymerizable monomers containingnitrile groups and hydrophilic groups to form grafted side chains,converting the nitrile groups in the grafted side chains into amidoximegroups, and conditioning the powder with an alkaline solution to obtaina powder-based adsorbent capable of complexing metal ions from seawateror other aqueous solution. Co-grafting is optionally performed while thepolyolefin powder is exposed to ionizing radiation, or alternativelyafter the polyolefin foam is exposed to ionizing radiation. Wherepolyacrylonitrile granules are provided, the steps of irradiating andco-grafting can be omitted as noted in Part II above.

VI. Applications Involving Fiber-Based, Foam-Based or Powder-BasedAdsorbents

The methods described in Parts I-V above can provide low cost,environmentally benign, high-surface area, reusable adsorbents that canbe used for rapidly and selectively extracting significant quantities ofvaluable and precious dissolved metals or for removing toxic dissolvedmetals from Earth's water sources including oceans, rivers, streams,lakes, ponds, hot springs, process water, wastewater, groundwater, stormwater and landfill/mine leachate. Principal applications for theseadsorbents include the extraction of valuable metals from seawater fornuclear energy and defense applications, as well as a broad range ofcommercial applications. Additionally, these adsorbents can be used forthe removal of toxic metals from contaminated water resources.Throughout the world, there are numerous contaminated sites that containa wide variety of toxic metals. This technology can provide aneconomical solution for cleaning up and reclaiming these sites andmaking them available for future use.

The adsorbents can be manufactured with a wide variety of otherfunctional groups besides the amidoxime group that a have a highaffinity for other selected metals. These applications can include: 1)the recover of rare earth metals for batteries (lanthanum), lasers(samarium, dysprosium), magnets (neodymium, holmium), and catalysts(lanthanum, cerium); 2) the recovery of other valuable and preciousmetals or toxic metals from oceans, rivers, streams, and mine run-off;3) the clean-up of oil spills; 4) the recovery of lithium for batteryapplications; 5) the collection of scandium from hot springs for use inalloys, high-intensity discharge lamps, tracing agents, and catalysts;6) the removal of toxic cadmium from scallop processing; and 7) use inion exchange membranes. In addition, because the adsorbents can betailored to preferentially adsorb metals at a range of loading levels,using concentrated metal solutions, they can be transformed intopolymer-metal complexes that potentially offer many intriguingapplications including: 1) polymer-supported metal complex catalysts; 2)battery and fuel cell membranes; and 3) anti-microbial materials (i.e.,silver).

The present invention is further illustrated in the following example,which is intended to be non-limiting.

EXAMPLE

A. Preparation of High Surface Area Fibers

Fiber-based adsorbents for the recovery of uranium from aqueoussolutions were formed using the below high surface area fibers availablefrom Hills, Inc.:

Approxi- mate fiber Fiber 1^(st) 2^(nd) Fiber # diameter shape polymerpolymer Ratio TR-714F-2  1 micron Round; 330 Dow 6850 6202 60/40 (#7)islands-in- LLDPE D PLA the-sea Islands Sea TR-714A-9  5 microns Round;37 Dow 6202 55/45 (#1) islands-in- 61800 D PLA the-sea LLDPE Sea IslandsTR-714A-5 12 microns Round; 6202 Dow 50/50 (#3) Sheath/Core D PLA 61800(Sheath) LLDPE (Core) TR-714E-1 14 microns British flag Dow 6202 50/50(#2) or flower 6850A D PLA shape LLDPE TR-714F-1 30 microns Hollow gearDow 6850 6202 40/60 (#8) (tooth-to- LLDPE D PLA tooth; 12-17 (core)(sheath) microns hollow size) TR-714F-1 18 microns Solid gear 6202 Dow6850 60/40 (#11) (tooth-to- (quasi- D PLA LLDPE tooth) trilobal)(Sheath) (core) TR-714F-2 17 microns Solid gear 6202 Dow 6850 60/40(#12) (tooth-to- (circular) D PLA (Core) tooth) (Sheath) ,

Prior to irradiation, each of the above fiber types were cut intoapproximately one inch lengths for ease of handling during processingand evaluation. Several of the high surface area fibers were made usingreadily dissolvable polylactic acid (PLA). The PLA was removed prior toirradiation by placing the fibers into flasks containing 200 ml oftetrahydrofuran (THF), drying at 50° C. for about two hours, and thenfiltering. This procedure was repeated a second time with new THF andallowed to sit overnight at 50° C., then filtered and dried at 50° C.under vacuum.

B. Irradiation and Co-Grafting of High Surface Area Polyethylene Fibers

The high surface area polyethylene fibers were put in nitrogen-inertedplastic bags and placed on top of a bed of dry ice and irradiated withelectron beams at 200 kGy. After irradiation, the fibers were immersedin a nitrogen-inerted, screw cap flask containing previously nitrogende-gassed acrylonitrile (AN) and methacrylic acid (MAA) graftingmonomers and dimethylsulfoxide (DMSO) and allowed to sit undisturbed for18 hours at 60° C. The composition of the grafting solutions includingthe ratio of DMSO solvent (or other solvent(s)) to AN and MAA monomers(or other monomer(s)) included: (a) 50AN/MAA-106 ml DMSO (50 wt. %), 101ml AN (35 wt. %), 34 ml MAA (15 wt. %); (b) 75AN/MAA-50 ml DMSO (25 wt.%), 142 ml AN (53 wt. %), 49 ml MAA (22 wt. %); and (c) 90AN/MAA-20 mlDMSO (10 wt. %), 172 ml AN (63 wt. %), 59 ml MAA (27 wt .%).

After grafting, the fibers were washed with dimethylformamide (DMF) andmethanol and dried at 50-60° C. under vacuum, then weighed to determinethe % degree of grafting (% DOG). % DOG is calculated by:

(weight after grafting-−weight before grafting)/(weight beforegrafting)*100

The % DOG results for some of the high surface area fibers are shown inFIG. 24, in which the sample number is listed, followed by the fibernumber and the grafting solution composition (e.g., 42H-#12 75AN/MAA).

C. Amidoxime Reaction and KOH Conditioning of High Surface Area Fibers

Approximately 150 mg of each type of high surface area grafted fiber wasadded to 250 ml flasks containing 75 ml of 10% hydroxylamine in 50/50(v/v) water/methanol solutions. After the flasks were sealed with ascrew cap they were allowed to sit undisturbed at 80° C. for 24 hours.The solutions were then drained and the fibers were physically separatedto maximize their surface area. Amidoximation of the fiber samples wasthen conducted a second time by placing the fiber materials in 250 mlflasks containing 75 ml of 10% hydroxylamine in 50/50 (v/v)water/methanol solution. After the flasks were sealed with screw capsthey were allowed to sit undisturbed at 80° C. for 24 hours. Thesolutions were then drained and the fiber samples were washed withde-ionized water and finally with methanol and dried under vacuum at 50°C.

After drying, approximately 15-30 mg of each fiber type was then addedto a 40 ml screw cap vial containing 15 ml of 2.5% KOH. After sealingthe vials they were shaken intermittently for 3 hours at 80° C. Thefibers were then filtered using a 1-inch diameter, Type 304 stainlesssteel woven wire disk (mesh/inch=80×80; wire diameter=0.0055-inch) andwashed three times with 40 ml of de-ionized water until a pH of about 7was attained, then kept wet until determination of the uraniumadsorption capacity was completed using an Inductively Coupled Plasmainstrument (ICP).

D. Determination of Uranium Adsorption Capacity on High Surface AreaFiber-Based Adsorbents using ICP Instrument

About 15-30 mg of each fiber-based adsorbent was placed in a samplecontainer containing de-ionized water, 6-7 ppm Uranyl nitratehexahydrate, 10,123 ppm of sodium ions, 15,529 ppm of chloride ions and140 ppm of bicarbonate ions with a pH of approximately 8. This containerwas then allowed to shake for 24 hours at 20-25° C. After shaking theadsorbent was filtered out and a sample of each solution was then putinto a plastic cap vial and the ICP instrument was used to determine thefinal uranium concentration in the solution. The ICP was also used todetermine the initial uranium solution concentration prior to adding theadsorbent sample. The uranium adsorption capacity(g-Uranium/kg-adsorbent) was calculated by subtracting the final uraniumsolution concentration from the initial uranium solution concentration.

FIG. 25 is a plot of the measured uranium adsorption capacity versus the% degree of grafting (% DOG) for the high surface area adsorbents formedabove and an existing non-woven adsorbent formed from round or circularcross-sectional, 20 micron diameter, high density polyethylene fibers.The measured uranium adsorption capacity for the existing nonwovenadsorbent was approximately 20 g-Uranium/kg-adsorbent, whereas the highsurface area adsorbents noted above ranged up to about 130g-Uranium/kg-adsorbent, which is about seven times higher in value. FIG.26 includes a table with the % DOG and measured Uranium adsorptioncapacities for the high surface area adsorbents and the existingnon-woven adsorbent. These results clearly demonstrate the significantincrease in the Uranium adsorption capacity of high surface areaadsorbents that are made from high surface area fibers in accordancewith the present embodiments.

The above description is that of current embodiments of the invention.Various alterations and changes can be made without departing from thespirit and broader aspects of the invention as defined in the appendedclaims, which are to be interpreted in accordance with the principles ofpatent law including the doctrine of equivalents. Any reference toelements in the singular, for example, using the articles “a,” “an,”“the,” or “said,” is not to be construed as limiting the element to thesingular.

1. A method comprising: providing a polymer fiber including asubstantially circular cross-section and defining a mean diameter ofless than 15 microns; exposing the polymer fiber to ionizing radiationto generate a reaction initiating a radical on the polymer fiber;co-grafting polymerizable monomers containing nitrile and hydrophilicgroups onto the polymer fiber to form grafted side chains; reacting thenitrile groups in the grafted side chains with a reagent to convert thenitrile groups into compounds adapted to complex metal ions; andconditioning the grafted polymer fiber with an alkaline solution toobtain a fiber-based adsorbent capable of complexing metal ions from anaqueous solution.
 2. The method according to claim 1 wherein the polymerfiber cross-section defines a mean diameter less than or equal to about1 micron.
 3. The method according to claim 1 wherein co-graftingpolymerizable monomers is performed while the polymer fiber is exposedto ionizing radiation.
 4. The method according to claim 1 whereinco-grafting polymerizable monomers is performed after the polymer fiberis exposed to ionizing radiation.
 5. The method according to claim 1wherein the metal ions include uranyl tricarbonate ion.
 6. The methodaccording to claim 1 wherein the metal ions are selected from the groupconsisting of uranium, titanium, vanadium, gold, platinum, palladium,silver, rare earth metals, mercury, chromium, cadmium, lead, cobalt,zinc, copper, iron and nickel
 7. The method according to claim 1 whereinthe polymer fiber is selected from a group consisting of polyethylene,polypropylene, polyamide, polyester, polyvinyl alcohol, polyvinylchloride, polytetrafluoroethylene-ethylene copolymer andpolyacrylonitrile.
 8. The method according to claim 1 wherein thenitrile group is selected from the group consisting of acrylonitrile,vinylidene cyanide, crotonnitrile, methacrylonitrile,chloroacrylonitrile, 2-cyanomethacrylate and 2-cyanoethylacrylate. 9.The method according to claim 1 wherein the hydrophilic group isselected from the group consisting of methacrylic acid, acrylic acid,itaconic acid, 2-hydroxyethyl methacrylate, 2-hydroxyethyl acrylate,allyl alcohol, polyethylene glycol acrylate, polyethylene glycolmethacrylate, polyethylene glycol diacrylate, polyethylene glycoldimethacrylate, N-vinylpyrrolidone, acrylamide, sulfonic acid group,carboxylic acid group, phenolic hydryoxyl group, phosphonic acid group.10. The method according to claim 1 wherein the reagent includeshydroxylamine.
 11. The method according to claim 1 wherein the reagentis selected from the group comprising hydroxylamine derivatives,hydrazine, hydrazine derivatives, N-methylhydroxylamine, acetohydroxamicacid, N-benzylhydroxylamine hydrochloride, hydroxyurea, tert-butyln-hydroxycarbamate, sym-diphenylhydrazine, methylhydrazine sulfate,phenylhydrazine and hydrochloride.
 12. The method according to claim 1wherein the monomers are contained within an aqueous-based emulsionincluding a surfactant.
 13. An adsorbent material comprising: aplurality of irradiated polymer fibers including co-grafted side chainsof amidoxime groups and hydrophilic groups adapted to complex metal ionsfrom an aqueous solution, wherein the irradiated polymer fibers includea substantially circular cross-section defining a mean diameter of lessthan 13 microns.
 14. The adsorbent material to claim 13 wherein thepolymer fiber cross- section includes a mean diameter less than or equalto about 1 micron.
 15. The adsorbent material of claim 13 wherein thepolymer fiber is selected from a group consisting of polyethylene,polypropylene, polyamide, polyester, polyvinyl alcohol, polyvinylchloride, polytetrafluoroethylene-ethylene copolymer, andpolyacrylonitrile.
 16. A method comprising: providing a polymer fiberincluding an outer surface defining a non-circular cross-section havinga plurality of projections for increasing the fiber surface area;exposing the polymer fiber to ionizing radiation and co-graftingpolymerizable monomers containing nitrile and hydrophilic groups to formgrafted side chains; reacting the nitrile groups in the grafted sidechains with a reagent to convert the nitrile groups into compoundsadapted to complex metal ions; and conditioning the irradiated, graftedpolymer fiber with an alkaline solution to obtain a fiber-basedadsorbent capable of adsorbing dissolved metals from an aqueoussolution.
 17. The method according to claim 16 wherein the non-circularcross-section is one of multi-lobed shaped, gear-shaped, wing-shaped andflower-shaped.
 18. The method according to claim 16 wherein co-graftingpolymerizable monomers is performed while the polymer fiber is exposedto ionizing radiation.
 19. The method according to claim 16 whereinco-grafting polymerizable monomers is performed after the polymer fiberis exposed to ionizing radiation.
 20. The method according to claim 16wherein the polymer is selected from a group consisting of polyethylene,polypropylene, polyamide, polyester, polyvinyl alcohol, polyvinylchloride, polytetrafluoroethylene-ethylene copolymer, andpolyacrylonitrile.
 21. The method according to claim 16 wherein thepolymer fiber forms at least one of a woven fabric, a braided fabric, aknitted fabric, and a nonwoven fabric.
 22. The method according to claim16 wherein the dissolved metals are selected from the group consistingof uranium, titanium, vanadium, gold, platinum, palladium, silver, rareearth metals, mercury, chromium, cadmium, lead, cobalt, zinc, copper,iron and nickel.
 23. The method according to claim 16 wherein thereagent is selected from the group consisting of hydroxylamine,hydroxylamine derivatives, hydrazine, hydrazine derivatives,N-methylhydroxylamine, acetohydroxamic acid, N-benzylhydroxylaminehydrochloride, hydroxyurea, tert-butyl n-hydroxycarbamate,sym-diphenylhydrazine, methylhydrazine sulfate, phenylhydrazine andhydrochloride.
 24. The method according to claim 16 wherein the monomersare contained within an aqueous-based emulsion including a surfactant.25. An adsorbent material comprising: a plurality of irradiated polymerfibers including co-grafted side chains of amidoxime groups andhydrophilic groups adapted to complex metal ions from an aqueoussolution, wherein the irradiated polymer fibers include an outer surfacedefining a non-circular cross-section having a plurality of projectionsfor increasing the fiber surface area per unit weight.
 26. The adsorbentmaterial of claim 25 wherein the cross-section includes an outerperiphery being one of multi-lobed-shaped, gear-shaped, winged-shapedand flower-shaped.
 27. The adsorbent material of claim 25 wherein thepolymer fiber is selected from a group consisting of polyethylene,polypropylene, polyamide, polyester, polyvinyl alcohol, polyvinylchloride, and polytetrafluoroethylene-ethylene copolymer.
 28. A methodfor forming a foam-based adsorbent for the recovery of metal ions froman aqueous solution, the method comprising: exposing a polymer foam toionizing radiation and co-grafting polymerizable monomers containingnitrile and hydrophilic groups to form grafted side chains; reacting thenitrile groups in the grafted side chains with hydroxylamine to convertthe nitrile groups into amidoxime groups; and conditioning the graftedpolymer foam with an alkaline solution.
 29. The method according toclaim 28 wherein the polymer foam is polyethylene foam or polypropylenefoam.
 30. The method according to claim 28 wherein the polymer foam issubstantially an open cell foam.
 31. The method according to claim 28wherein the polymer foam is substantially a closed cell foam.
 32. Themethod according to claim 28 wherein the reagent is selected from thegroup consisting of hydroxylamine, hydroxylamine derivatives, hydrazine,hydrazine derivatives, N-methylhydroxylamine, acetohydroxamic acid,N-benzylhydroxylamine hydrochloride, hydroxyurea, tert-butyln-hydroxycarbamate, sym-diphenylhydrazine, methylhydrazine sulfate,phenylhydrazine and hydrochloride.
 33. A method for forming apowder-based adsorbent for the recovery of metal ions from an aqueoussolution, the method comprising: exposing a powder including polymergranules to ionizing radiation; co-grafting polymerizable monomerscontaining nitrile and hydrophilic groups onto the irradiated powder toform grafted side chains; reacting the nitrile groups in the graftedside chains with hydroxylamine to convert the nitrile groups intoamidoxime groups; and conditioning the powder with an alkaline solution.34. The method according to claim 33 wherein the polymer granules have amean diameter of less than approximately 500 microns.
 35. The methodaccording to claim 33 wherein the polymer granules have a mean diameterof less than approximately 50 microns.
 36. The method according to claim33 wherein the polymer granules have a mean diameter of less thanapproximately 5 microns.
 37. The method according to claim 33 whereinthe polymer granules are formed from ground polyolefin fibers.
 38. Themethod according to claim 33 wherein the polymer granules arepolyethylene, polypropylene, or polyacrylonitrile.
 39. The methodaccording to claim 33 wherein the reagent is selected from the groupconsisting of hydroxylamine, hydroxylamine derivatives, hydrazine,hydrazine derivatives, N-methylhydroxylamine, acetohydroxamic acid,N-benzylhydroxylamine hydrochloride, hydroxyurea, tert-butyln-hydroxycarbamate, sym-diphenylhydrazine, methylhydrazine sulfate,phenylhydrazine and hydrochloride.